Shouldn't the 1940 cooling system run a zero pressure radiator with a vent tube in the top tank? If someone has put a pressure cap of any capacity on it, that's a mistake. It's also what is causing the "pinhole" leak. There should be NO pressure at all in that system, and if you run a thermostat, don't go above a 160 degree unit.
Bottom line- - - -you need to get the radiator repaired. A "small" leak is like being a little bit pregnant- - - - -the situation isn't going to improve. You should also do a "block test" where a colored liquid is used to detect exhaust gas in the coolant. If the indicator liquid turns from blue to green (small leak) or to yellow (BIG leak) it's time for a head gasket change or worse, the head is cracked.
Paragraphs and punctuation are your friends. Some of us old geezers get out of breath reading posts like yours!
Jerry
